A sensitive radioreceptor assay for the determination of glycopyrrolate concentrations in human plasma, urine and cerebrospinal fluid (CSF) is described. The applicability of the assay for kinetic studies in human was studied by determining the plasma concentrations and the renal excretion in three gynaecological surgical patients, who received 8 micrograms/kg of glycopyrrolate as a premedication intramuscularly. Tritiated N-methyl scopolamine was used to label the muscarinic cholinergic receptors in the membrane preparation obtained from the rat brain. The limit of detection of the assay was 70 ng/l in plasma, 2 micrograms/l in urine and 140 ng/l in CSF. There was no evidence of cross-reactivity of glycopyrrolate derivatives in clinical concentrations. A very rapid absorption was found with a mean maximum plasma concentration (Cmax) of 14.26 (range 12.02-16.97) micrograms/l and mean Tmax (time to Cmax) of 13.3 (range 10-15) min. and almost 50% of the dose administered was excreted into the urine within 3 hr. The CSF levels of glycopyrrolate were under detection limit. It is concluded that the sensitivity of the method is sufficient for pharmacokinetic studies of glycopyrrolate after therapeutic dosing.